Documentation: Add SMMSTORE documentation

This documents the smmstore API.

Change-Id: I992c04c0cf9b3f03755cf3fede2c82c6471a5ef4
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37243
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

Documentation/drivers/index.md

@@ -5,3 +5,4 @@ and plugin devices, significantly reducing integration complexity and
 they allow to easily reuse existing code accross platforms.
 
 * [IPMI KCS](ipmi_kcs.md)
+* [SMMSTORE](smmstore.md)

Documentation/drivers/smmstore.md

@@ -0,0 +1,123 @@
+# SMM based flash storage driver
+
+This documents the API exposed by the x86 system management based
+storage driver.
+
+## SMMSTORE
+
+SMMSTORE is a SMM mediated driver to read from, write to and erase a
+predefined region in flash. It can be enabled by setting
+`CONFIG_SMMSTORE=y` in menuconfig.
+
+This can be used by the OS or the payload to implement persistent
+storage to hold for instance configuration data, without needing
+to implement a (platform specific) storage driver in the payload
+itself.
+
+The API provides append-only semantics for key/value pairs.
+
+## API
+
+### Storage region
+
+By default SMMSTORE will operate on a separate FMAP region called
+`SMMSTORE`. The default generated FMAP will include such a region.
+On systems with a locked FMAP, e.g. in an existing VBOOT setup
+with a locked RO region, the option exists to add a cbfsfile
+called `smm_store` in the `RW_LEGACY` (if CHROMEOS) or in the
+`COREBOOT` FMAP regions. It is recommended for new builds using
+a handcrafted FMD that intend to make use of SMMSTORE to include a
+sufficiently large `SMMSTORE` FMAP region. It is recommended to
+align the `SMMSTORE` region to 64KiB for the largest flash erase
+op compatibility.
+
+When a default generated FMAP is used the size of the FMAP region
+is equal to `CONFIG_SMMSTORE_SIZE`. UEFI payloads expect at least
+64KiB. Given that the current implementation lacks a way to rewrite
+key-value pairs at least a multiple of this is recommended.
+
+### generating the SMI
+
+SMMSTORE is called via an SMI, which is generated via a write to the
+IO port defined in the smi_cmd entry of the FADT ACPI table. `%al`
+contains `APM_CNT_SMMSTORE=0xed` and is written to the smi_cmd IO
+port. `%ah` contains the SMMSTORE command. `%ebx` contains the
+parameter buffer to the SMMSTORE command.
+
+### Return values
+
+If a command succeeds, SMMSTORE will return with
+`SMMSTORE_RET_SUCCESS=0` on `%eax`. On failure SMMSTORE will return
+`SMMSTORE_RET_FAILURE=1`. For unsupported SMMSTORE commands
+`SMMSTORE_REG_UNSUPPORTED=2` is returned.
+
+**NOTE1**: The caller **must** check the return value and should make
+no assumption on the returned data if `%eax` does not contain
+`SMMSTORE_RET_SUCCESS`.
+
+**NOTE2**: If the SMI returns without changing `%ax` assume that the
+SMMSTORE feature is not installed.
+
+### Calling arguments
+
+SMMSTORE supports 3 subcommands that are passed via `%ah`, the additional
+calling arguments are passed via `%ebx`.
+
+**NOTE**: The size of the struct entries are in the native word size of
+smihandler. This means 32 bits in almost all cases.
+
+
+#### - SMMSTORE_CMD_CLEAR = 1
+
+This clears the `SMMSTORE` storage region. The argument in `%ebx` is
+unused.
+
+#### - SMMSTORE_CMD_READ = 2
+
+The additional parameter buffer `%ebx` contains a pointer to
+the following struct:
+
+```C
+struct smmstore_params_read {
+	void *buf;
+	ssize_t bufsize;
+};
+```
+
+INPUT:
+- `buf`: is a pointer to where the data needs to be read
+- `bufsize`: is the size of the buffer
+
+OUTPUT:
+- `buf`
+- `bufsize`: returns the amount of data that has actually been read.
+
+#### - SMMSTORE_CMD_APPEND = 3
+
+SMMSTORE takes a key-value approach to appending data. key-value pairs
+are never updated, they are always appended. It is up to the caller to
+walk through the key-value pairs after reading SMMSTORE to find the
+latest one.
+
+The additional parameter buffer `%ebx` contains a pointer to
+the following struct:
+
+```C
+struct smmstore_params_append {
+	void *key;
+	size_t keysize;
+	void *val;
+	size_t valsize;
+};
+```
+
+INPUT:
+- `key`: pointer to the key data
+- `keysize`: size of the key data
+- `val`: pointer to the value data
+- `valsize`: size of the value data
+
+## External links
+
+* [A Tour Beyond BIOS Implementing UEFI Authenticated Variables in SMM with EDKI](https://software.intel.com/sites/default/files/managed/cf/ea/a_tour_beyond_bios_implementing_uefi_authenticated_variables_in_smm_with_edkii.pdf)
+Note, this differs significantly from coreboot's implementation.